Researchers compared dietary intake, physical activity, and BMI across different age groups. They found that when eating the same amount of food, 20-somethings nowadays gain weight more readily than their peers 35 years ago. The average BMI of a young person was 10 percent more in 2008 than in 1971. Researchers don’t blame overeating alone; they also point to environmental factors.

"This is because weight management is actually much more complex than just 'energy in' versus 'energy out'," said Jennifer Kuk, a professor in the School of Kinesiology and Health Science at York University in Ontario, in a press release. "That's similar to saying your investment account balance is simply your deposits subtracting your withdrawals and not accounting for all the other things that affect your balance like stock market fluctuations, bank fees or currency exchange rates."